mpeg2dec: use default pad accept-caps handling
mpeg2dec: use default pad accept-caps handling

Avoids useless check of downstream caps when handling an
accept-caps query

GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,Kurento/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Kurento/gst-plugins-ugly,Kurento/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GStreamer/gst-plugins-ugly